1. Field of the Invention
The invention relates to a device for melting and/or refining inorganic compounds, particularly broken glass or so-called batches as a starting material for glass.
2. Description of the Related Art
Such devices comprise a container for receiving the material to be heated, i.e. the broken glass or batch, and also a high-frequency system for heating the material by means of supplying high-frequency energy to the material in the container. The high-frequency system comprises a high-frequency oscillation circuit having an induction coil looping around the container and an oscillator.
Generally, the container is a crucible which is transparent with respect to the high-frequency radiation. It may consist of ceramic material, quartz glass or slotted metallic material. The glass or the melt, respectively, initially has to be preheated using means other than the high-frequency energy so as to achieve a certain minimum conductivity for the material. Once the connection temperature is reached the energy may be supplied via irradiating high-frequency energy.
The walls of the crucible have to be cooled. This is achieved by means of heat dissipation or by active air- or water-cooling, for example. A relatively cold crust of the characteristic material, thus the glass, is then formed in the wall area.
Devices of the above type are known in the art based on the following, for example:
WO 92/15531
DE 33 16 546 C1
U.S. Pat. No. 4,780,121 A1
JP 57-95834.
The device according to WO 92/15531 operates continuously. The energy for heating the crucible is supplied by one single high-frequency generator. This is also the case in devices according to DE 33 16 546 C1 and U.S. Pat. No. 4,780,121 A1.
The device according to JP 57-95834 has a crucible having a relatively large axial expansion. In order to operate the crucible with varying temperature ranges two high-frequency generators are provided each of which operates an inductor. As a result of a varying output from the generators the temperature profile can be set along the crucible.
The above mentioned electrical units are highly stressed in view of the high energy density. Therefore, the possibility that one of the units will fail must be taken into consideration. This applies particularly to continuous operation, such as a device according to WO 92/15531. The stress on the system is magnified because of the continuous operation compared to the batch operation.
Accordingly, the risk that one of the units will fail is particularly high in continuous operation. Furthermore, the consequences of a breakdown are particularly serious. For example, the following may occur: A breakdown in the energy supply will result in a temperature drop which decreases the conductivity of the melt. From a certain temperature on the high-frequency energy can no longer be connected. The melt will solidify. Once the continuous glass flow has stopped, however, reconnecting by means of high frequency is not possible after the defect has been repaired because the glass is cold and thus it has a low electric conductivity and is unable to absorb high-frequency energy.
Because of the above circumstances in practical application the industry has refrained from using devices of the above described type, or systems equipped with said devices, for continuous operation.
The object of the invention is to design a device of the above described type such that the risks of continuous operation are minimized or largely eliminated.
The present invention provides a device for melting and refining glass and ceramic compounds, and also for heating the compounds for crystal growing. The device includes a crucible for receiving the compound to be heated and a high-frequency system for supplying high frequency energy to the compound. The high-frequency system includes a first self-exciting oscillation circuit having an induction coil looping around the crucible and a capacitor connectable to the induction coil.
The high-frequency system further includes at least a second self-exciting oscillator that includes the induction coil and a second capacitor connectable to the induction coil. Both the first and second oscillators are connectable with the induction coil individually or jointly and are operable as an oscillation circuit without the necessity of a change of the remaining oscillator circuits. The high-frequency system may further include a third self-exciting oscillator. An advantage of this arrangement is that a failed oscillator can be removed from the high-frequency system while the remaining oscillators continue operating alone.